ICU-22872 Fix BigDecimal scale in DecimalFormat.parse()

[aryanraj45]

Fix BigDecimal scale preservation in DecimalFormat.parse()

Root cause: The internal BCD (Binary Coded Decimal) representation doesn't store trailing zeros. When parsing decimal numbers like "0.0" or "1.00", the BigDecimal scale was incorrectly set to 0 instead of preserving the number of decimal places from the input string.

Solution:

• Added fractionDigitCount field to ParsedNumber to track the number of digits after the decimal point during parsing
• Modified getNumber() to apply the correct scale to the resulting BigDecimal when it would otherwise be too small
• This scale adjustment is skipped for numbers with exponents (e.g., "1.0E3") since the exponent changes the effective decimal position

Changes:

• ParsedNumber.java: Added fractionDigitCount field and scale adjustment logic in getNumber()
• DecimalMatcher.java: Set fractionDigitCount when decimal separator is encountered
• NumberFormatTest.java: Added TestParseBigDecimalScale test case covering inputs like "0.0", "1.00", "123.450"

Testing:

• All existing tests pass (2890 tests, 0 failures)
• New unit test validates scale preservation for various decimal inputs
• Verified that scientific notation is handled correctly (no scale adjustment when exponent is present)

Related: https://github.com/unicode-org/icu/blob/main/CONTRIBUTING.md

Checklist

• Required: Issue filed: ICU-22872
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-22872 Fix BigDecimal scale in DecimalFormat.parse()"
• Required: Each commit message must be prefixed with a JIRA Issue number. Example: "ICU-22872 Fix BigDecimal scale in DecimalFormat.parse()"
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable
• Approver: Feel free to merge on my behalf